The freezing‒thawing index and permafrost extent in pan-Arctic experienced rapid changes following the global warming hiatus

Global ground surface temperatures experienced a rapid increase following the end of the global warming hiatus in 2013. The rapid temperature increase has potential to drive changes in FDD/TDD and permafrost extent in pan-Arctic. In this study, the temporal and spatial trends of air freezing‒thawing index (AFDD/ATDD) and ground surface freezing‒thawing index (GFDD/GTDD) in pan-Arctic from 2003 to 2023 were analyzed, with a particular focus on the changes between the decades preceding and following 2013. The changes in permafrost extent were also analyzed. The results indicate that from 2003 to 2023, the AFDD and GFDD significantly (p < 0.05) decreased at rates of 11.07 and 5.34 °C d per year, while the ATDD and GTDD significantly (p < 0.05) increased at rates of 7.69 and 4.34 °C d per year, respectively. The FDD/TDD experienced rapid changes after the global warming hiatus, with the decreasing rates in AFDD and GFDD intensifying after 2013 to 15.75 °C d per year and 6.27 °C d per year, respectively, and increasing rates in ATDD and GTDD intensifying after 2013 to 15.88 °C d per year and 8.50 °C d per year, respectively. The permafrost area in pan-Arctic experienced a decline from 13.4 × 10⁶ km² in 2003–2013 to 12.51 × 10⁶ km² in 2014–2023, representing a decadal reduction of 0.78 × 10⁶ km² (6.64%). The rapid decadal reduction in permafrost extent surpassed the decadal changes projected by historical trend since 1901, despite a slight expansion in southern Canada. The results provide novel insights into recent changes in FDD/TDD and permafrost extent.